While this question seems to be a difficult one to answer if one only looks at Prophet Muhammad, peace and blessings be upon him, from an isolated angle, because it would then seem that the Prophet have made up this law to conveniently self-serve himself, but in reality, GOD Almighty had put a similar Law to this, thousands of years before Prophet Muhammad, in the Bible.

First let us look at what Allah Almighty Said in the Noble Quran:

"The Prophet is closer to the Believers than their own selves, and his wives are their mothers. Blood-relations among each other have closer personal ties, in the Decree of God. Than (the Brotherhood of) Believers and Muhajirs: nevertheless do ye what is just to your closest friends: such is the writing in the Decree (of God).  (The Noble Quran, 33:6)"

"Prohibited to you (For marriage) are:- Your mothers,.....(The Noble Quran, 4:23)"

Now let us look at the following verses from the Bible:

Leviticus 21:5-8
5 " 'Priests must not shave their heads or shave off the edges of their beards or cut their bodies.
6 They must be holy to their God and must not profane the name of their God. Because they present the offerings made to the LORD by fire, the food of their God, they are to be holy.
7 " 'They must not marry women defiled by prostitution or divorced from their husbands, because priests are holy to their God.
8 Regard them as holy, because they offer up the food of your God. Consider them holy, because I the LORD am holy—I who make you holy.

Leviticus 21:13-15
13 " 'The woman he marries must be a virgin.
14 He must not marry a widow, a divorced woman, or a woman defiled by prostitution, but only a virgin from his own people,
15 so he will not defile his offspring among his people. I am the LORD, who makes him holy.' "

Ezekiel 44:21-22
21 No priest is to drink wine when he enters the inner court.
22 They must not marry widows or divorced women; they may marry only virgins of Israelite descent or widows of priests.
 

Prophet Muhammad's Wives, peace, mercy and blessings be upon all of them, are like those Priests.  They are different and higher than everyone else.  As we clearly see, the Bible too contains special laws and special treatment for specific people who are considered higher and different than everyone else from the ordinary public.  Even Muslims as a whole are considered special and are raised above everyone else in the Noble Quran:

"Do not marry unbelieving women (idolaters), until they believe: A slave woman who believes is better than an unbelieving woman, even though she allures you. Nor marry (your girls) to unbelievers until they believe: A man slave who believes is better than an unbeliever, even though he allures you. Unbelievers do (but) beckon you to the Fire. But God beckons by His Grace to the Garden (of bliss) and forgiveness, and makes His Signs clear to mankind: That they may celebrate His praise.  (The Noble Quran, 2:221)"

The anti-Islamics viciously use these types of controversial topics to attack Islam and to try to prove that somehow Prophet Muhammad, peace, mercy and blessings be upon him, was not truthful and that he employed the Noble Quran to his personal service.  The next time you run into one, especially in an audio chat/debate Islam-bashing rooms, expose their ignorance by using the above Verses.

back